Common hybrid working machines drive a hydraulic pump with the output of an engine (internal combustion engine) and do work with the generated hydraulic pressure. The engine is assisted with an electric motor to be driven with efficiency. The electric motor is driven mainly with electric power from a battery. The battery is allowed to be charged and discharged. In the case of assisting the engine, the battery is discharged to supply electric power to the electric motor. On the other hand, in the case of not assisting the engine, the battery is charged with electric power from a generator driven by the engine or regenerated electric power from a hydraulic load. Thereby, it is made possible to assist the electric motor by keeping the battery constantly charged to a certain level.
Thus, in hybrid working machines, it is possible to assist the engine with the electric motor. Therefore, the maximum output of the engine is reduced so that the engine may be reduced in size. If an output greater than the maximum output of the engine is required of the hydraulic pump, the request is met by assisting the engine with the electric motor.
By using a motor generator for the electric motor, the electric motor function and the generator function may be combined into one. In this case, it is necessary to control whether to execute the assist function as an electric motor or to execute the power generating function as a generator.
Therefore, it has been proposed to determine the output of the hydraulic pump by operations and to perform switching control over whether to cause a motor generator to operate as an electric motor or to operate as a generator by comparing the determined hydraulic pump output with a threshold. (See, for example, Patent Document 1.)
Further, in hybrid working machines, a variable displacement hydraulic pump is often used in order to allow a generated hydraulic pressure to vary in accordance with a hydraulic load's request. (See, for example, Patent Document 2.)